1. Field of the Invention
The present invention relates generally optical systems. The present invention relates more particularly to micro electromechanical devices (MEMs) that may be used in optical systems.
2. Description of the Background Art
Arrays of light-modulating elements may be applied to communications systems. The light-modulating elements may comprise, for example, GRATING LIGHT VALVE (GLV) elements. For example, the arrays may be used as a micro electromechanical system (MEMS) for use in optical networks. In another application, a two-dimensional projection image may also be formed by using one or more linear arrays of light-modulating elements. In such display systems, the linear array modulates an incident light beam to display pixels along a column (or, alternatively, a row) of the two-dimensional (2D) image. A scanning system is used to move the column across the screen such that each light-modulating element Is able to generate a row of the 2D image. In this way, the entire 2D image is displayed. There are also other applications of light-modulating elements.
Publications describing GLV devices and their applications include: xe2x80x9cThe Grating Light Valve: Revolutionizing Display Technology,xe2x80x9d by D. M. Bloom, Projection Displays III Symposium, SPIE Proceedings, Volume 3013, San Jose, Calif., February 1997; xe2x80x9cGrating Light Valve Technology: Update and Novel Applications,xe2x80x9d by D. T. Amm and R. W. Corrigan of Silicon Light Machines in Sunnyvale, Calif., a paper presented at the Society for Information Display Symposium, May 19, 1998, Anaheim, Calif.; xe2x80x9cOptical Performance of the Grating Light Valve Technology,xe2x80x9d David T. Amm and Robert W. Corrigan of Silicon Light Machines, a paper presented at Photonics West-Electronics Imaging, 1999; xe2x80x9cCalibration of a Scanned Linear Grating Light Valve Projection System,xe2x80x9d R. W. Corrigan, D. T. Amm, P. A. Alioshin, B. Staker, D. A. LeHoty, K. P. Gross, and B. R. Lang, a paper presented at the Society for Information Display Symposium, May 18, 1999, San Jose, Calif.; xe2x80x9cAn Alternative Architecture for High Performance Display,xe2x80x9d R. W. Corrigan, B. R. Lang, D. A. LeHoty, and P. A. Alioshin of Silicon Light Machines, a paper presented at the 141 st SMPTE Technical Conference and Exhibition, Nov. 20, 1999, New York, N.Y.; xe2x80x9cBreakthrough MEMS Component Technology for Optical Networks,xe2x80x9d Robert Corrigan, Randy Cook, and Olivier Favotte, Silicon Light Machinesxe2x80x94Grating Light Valve Technology Brief, 2001; and U.S. Pat. No. 6,215,579, entitled xe2x80x9cMethod and Apparatus for Modulating an Incident Light Beam for Forming a Two-Dimensional Image,xe2x80x9d and assigned at issuance to Silicon Light Machines. Each of the above-mentioned publications is hereby incorporated by reference in its entirety.
FIG. 1 is a diagram depicting the reflective and diffractive operational states of a GRATING LIGHT VALVE (GLV) element. The left side of the diagram depicts the reflective (dark) state, while the right side of the diagram depicts the diffractive (bright) state.
In the example illustrated in FIG. 1, the substrate may comprise a silicon substrate with oxide (for example, about 5000 angstroms thick) overlaying the silicon, and tungsten (for example, about 1000 angstroms thick) overlaying the oxide. The reflective members lie above the tungsten with an air space there between. For example, three pairs of reflective members (i.e. six members) are shown. The reflective members may, for example, comprise reflective ribbons comprising nitride (for example, about 1000 angstroms thick) with a reflective layer of aluminum (for example, about 500 angstroms thick) on the nitride. Incident light is beamed onto the reflective members. The incident light beam may be at a perpendicular angle to the plane of the substrate.
In the reflective or dark state (left side), all the reflective members are in the same plane, and the incident light is reflected from the surfaces of the members. This reflective state may be called the dark state because it may be used to produce a dark spot (dark pixel) in a projection display system. Such a dark pixel may be produced by blocking the light that is reflected back along the same path as the incident light.
In the diffractive or bright state (right side), alternate ones of the reflective members are deflected downward. This results in the diffraction of the incident light in a direction that is at an angle to the path of the incident light. This reflective state may be called the bright state because it may be used to produce a bright spot (bright pixel) in a projection display system. Such a bright pixel may be produced because the angularly reflected light is not blocked. As discussed further below, the optical response of the element depends on the amount of downward deflection of the alternate members.
FIG. 2 is an illustration depicting a GLV element comprising pairs of fixed and movable ribbons. As depicted in FIG. 2, the GLV element may include pairs of reflective ribbons, each pair having one fixed and one movable ribbon.
FIG. 3 is a diagram depicting deflections of reflective members for a GLV element in a diffractive state. The GLV element comprises a plurality of reflective members. The reflective members comprise alternating bias members 304 and active member 306. In the example illustrated, the GLV element includes three pairs of reflective members (i.e. six of them).
In the diffractive state, the reflective members are controllably arranged in an alternating configuration at two heights from a common electrode 308, where bias members 304 are at a first height and active (movable) members 306 are at a second height. The bias members 304 may be fixed ribbons. The active members 306 may be movable ribbons pulled down by application of a voltage. The voltage may be applied with respect to the common electrode 308. As illustrated in FIG. 3, the incident light beam 310 impinges upon the element at an angle perpendicular to the grating plane. Diffracted light 312 travels away from the element.
The difference between first and second heights may be defined as the deflection distance xcex3. The amount of the deflection xcex3 may be varied by application of different voltages to control the amount of incident light reflected from the element. When xcex3 is near zero, the element would be near a maximally reflective state. When xcex3 is near xcex/4, where xcex is the wavelength of the incident light, the element would be near a maximally diffractive state.
FIG. 4 is a graph illustrating a unipolar electro-optic response for a first order diffraction from a GLV element. The graph shows intensity of light (in arbitrary units) from the first order diffraction versus voltage. The higher the voltage is, the larger is the displacement xcex3 of the element. Depending on the voltage applied to the active members, the light intensity varies. For the most part, the higher the applied voltage, the higher the light intensity for the first order diffraction. (This relationship reverses for sufficiently high voltages where the light intensity reduces with higher voltages, and hence the downward slope of the graph at the far right.)
FIG. 5A is a diagram illustrating elements of a light-modulating array being utilized as variable optical attenuators (VOAs) for a dynamic gain equalizer (DGE). The multiple VOA devices shown in FIG. 5B may correspond to GLV devices in a linear array. The input light signals of various wavelengths (xcex1,xcex2,xcex3, . . . xcexN) may originally have various amplitude levels. The multiple wavelengths are dispersed by the dispersive element onto the multiple VOA devices. Each wavelength may be attenuated a variable amount by the VOA device on which it impinges. The wavelengths may then be combined by the combining element and subsequently amplified, for example, by an erbium doped fiber amplifier (EDFA). As a result, the amplitudes (gain levels) of the various wavelengths may be equalized.
FIG. 5B is a top view depicting a projection display system 500 utilizing a light-modulating array. The system 500 includes one or more light sources 502, one or more arrays of light-modulating elements 504, an optical scanner 506, and a screen 508. The figure is for explanatory purposes and is not necessarily accurate in scale or angles.
The light source 502 may comprise one or more laser light sources. Three laser light sources of different colors may be utilized for a color display system. The light-modulating array 504 may comprise an array of GLV elements (also called GLV xe2x80x9cpixelsxe2x80x9d) described above. Each light source 502 may illuminate a light-modulating array 504. Each element of an array 504 modulates the light incident on it to control the amount of light diffracted therefrom. The diffracted light from the elements of the array 504 is then directed to the optical scanner 506.
The optical scanner 506 is used to move the column (or row) of light across the screen 508. Various types of scanners 506 may be used. For example, galvonometer-based scanners, resonant scanners, polygon scanners, rotating prisms, or other types of scanners may be used. A drive signal is applied to the scanner to control (xe2x80x9cdrivexe2x80x9d) the movement of the column (or row) of light. For example, to achieve a progressive scan of the column across the screen (e.g., from left to right), a sawtooth drive signal may be used.
One disadvantageous aspect of using GLV and other MEMS technology relates to changes in device performance as a function of time. Whether used in a communication or other system, the response function of a GLV element and other MEMS devices has been observed to change over time. Such time-dependent changes can lead to unpredictable behavior and thus may limit the applications for GLV elements and other MEMS devices.
One embodiment of the invention relates to a method for adaptive bipolar operation of a micro electromechanical (MEM) device. The method includes driving the MEM device in a first polarity, determining when an offset passes a first threshold, and switching from the first polarity to a second polarity when the offset passes the first threshold.
Another embodiment of the invention relates to an apparatus for adaptive bipolar operation of a MEM device. The apparatus includes a first look-up table (LUT) for use in driving the MEM device in a first polarity, a circuit for determining when an offset passes a first threshold, and a polarity switch for switching from the first polarity to a second polarity when the offset passes the first threshold.
These and other features of the present invention will be readily apparent to persons of ordinary skill in the art upon reading the entirety of this disclosure, which includes the accompanying drawings and claims.